Please wait for me . . . . . . . . 8 o'clock.

The correct preposition to use in this sentence is "till". The sentence should read, "Please wait for me till 8 o'clock." The word "till" is used to indicate the point in time up to which the action is expected to continue.

Option B: by is not the correct answer as it does not convey the same meaning as "till." "By" indicates a point in time before which the action should be completed.

Option C: since is not the correct answer as it is used to indicate a starting point in time, not the duration of waiting.

Option D: to is not the correct answer as it does not convey the intended meaning of waiting until 8 o'clock.

Given the options provided, the correct answer is Option A: till.